1. Field of the Invention
The present invention relates to a technical field of rotation drive apparatuses, and more particularly to a technique for preventing vibrations which are created when a composite rotor which is imbalanced in terms of weight is rotated and reducing the thickness of an automatic aligning mechanism.
2. Prior Art
A main portion of industrial machines, home electronic products and computers has a rotative member which is rotated by a motor or the like. For example, a disc drive unit (a rotating mechanism) is structured in such a manner that an optical pickup unit or a magnetic head unit records and/or reproduces an information signal while an optical disc or a magneto-optical disc, which is a signal recording medium, is rotated. The optical disc, which is adapted to the disc drive unit of the foregoing type and which is rotated by the disc rotation drive apparatus, is included as a portion of the composite rotor.
The disc drive unit has an optical pickup unit for irradiating an optical disc, which is rotated by the disc rotation drive apparatus, with a laser beam. The disc rotation drive apparatus has a disc table on which an optical disc is mounted and a spindle motor for rotating the disc table.
The optical pickup unit has a light source for irradiating the optical disc with a laser beam and a photodetector for receiving the laser beam reflected by the signal recording surface of the optical disc. The optical pickup unit and the disc rotation drive apparatus are mounted on, for example, a sub-chassis. The sub-chassis supports the optical pickup unit in such a manner that the optical pickup unit is able to move in directions in which the optical pickup unit is brought into contact with the disc rotation drive apparatus, which fixedly supports the sub-chassis, and the same is moved away from the disc rotation drive apparatus.
The disc drive unit having the above-mentioned structure is arranged in such a manner that the optical disc, which is rotated by the spindle motor, is irradiated with a laser beam emitted from the optical pickup unit to record and/or reproduce an information signal.
The recording disc, such as the optical disc, sometimes encounters a weight imbalance when the recording disc is manufactured. If a recording disc having the weight imbalance is rotated, the center of rotation and the center of gravity do not coincide with each other. In this case, the recording disc is vibrated together with the disc table. If vibrations of the foregoing type are created, focusing and tracking with respect to the signal recording surface of the recording disc which are performed by the optical pickup unit become unsatisfactory. What is worse, the magnetic head apparatus cannot satisfactorily follow the recording disc.
The amount of the created imbalance of the recording disc varies depending on the recording disc. If the thickness or the density of the substrate of the recording disc is nonuniform, the center of gravity of the recording disc is not positioned at the center of the recording disc. As a result, each recording disc encounters imbalance in terms of the weight. For example, the CD-ROM (Compact Disc Read Only Memory) is permitted to have an amount of imbalance of 1 g.cm in accordance with the regulated standard therefor.
Recently, data has been recorded on the recording disc or reproduced from the same at high speed. Since the rotational speed has been raised, there arises a problem in that the composite rotor (including the recording disc) is vibrated excessively.
Therefore, the inventor of the present invention has suggested a rotation control apparatus which has a balancing member disposed rotatively with respect to the disc table. During rotations of the disc table, the balancing member of the rotation control apparatus is moved to a position at which the imbalance of the recording disc is suspended so that the balance is kept. As a result, the recording disc, the disc table and the balancing member are brought to a state in which the center of gravity and the center of rotation coincide with each other. As a result, vibration is not created so that the recording disc is consistantly rotated at high speed.
The rotation control apparatus having the foregoing balancing member must be structured in such a manner that the operation for making the center of gravity of the overall body including the composite rotor consisting of the recording disc and the disc table and the rotation center to coincide with each other, that is, the aligning operation can effectively be performed. To enable this, the relative movement of the balancing member with respect to the rotor must be performed smoothly. That is, frictional force generated between the balancing member and the rotor when the balancing member is rotated with respect to the rotor must be reduced to a minimum.
If the frictional force generated between the balancing member and the rotor is reduced, the balancing member separates from the rotor when the stopped rotor starts rotating. Thus, the balancing member cannot start rotating together with the rotor. The aligning operation is performed when the balancing member synchronizes with the rotor and rotates at the same speed as that of the rotor. Therefore, the balancing member must start rotating when the rotor starts rotating to cause the aligning operation to quickly be performed. If the balancing member is brought to a position at which it generates imbalance when the rotor stops moving, there is apprehension that large vibrations may occur in a period between the start of rotation of the rotor and the execution of the aligning operation.